The role and molecular mechanism of action of growth factors is an exciting area of biomedical research. The involvement of growth factors in the pathogenetic mechanisms of human disease has increasingly been postulated. Yet the superb tissue culture studies on which the field is based need to be buttressed by physiologic studies that define a role for the autocrine, paracrine or endocrine secretion of growth factors. We have demonstrated that the EGF receptor is down regulated during a well studied model of in vivo growth control, rat liver regeneration after partial hepatectomy. Continued funding is requested to allow detailed assessment of te potential role of EGF-EGF receptor interaction during liver regeneration. A complex picture of the regulation of EGF receptor affinity, number and internalization as emerging from tissue culture studies. Other pharmacologic and humoral agents can alter EGFR-EGF receptor interaction. EGF receptor modifications may be mediated by the intracellular messengers of humoral agents that do not bind directly to the EGF receptor. Thus other constituents of the complex humoral signal that regulates regeneration (e.g., insulin, glucagon, norepinephrine, vasopressin, etc.) may indirectly modify the EGF receptor. We will address these issues by comparing in vivo and in vitro findings in liver and hepatic cells. The following aims are proposed. 1) To analyze total cellular EGF receptor content after partial hepatectomy and sham operation to determine the earliest times at which changes occur. 2) To characterize the mechanism of EGF receptor down regulation during liver regeneration by studying the biosynthesis and degradation of radio-labeled receptor in vivo. Several antibodies specific for the extracellular and cytoplasmic domains of the rat EGF receptor have been raised and will allow analysis by immunoprecipitation. 3) To study the biosynthesis of EGF receptor mRNA during regeneration. Several cDNA probes from the 5' and 3' regions of the human EGF receptor are available and will be tested. 4) To identify the substances responsible for down-regulation of the EGF receptor during regeneration. 5) To determine whether hormones that potentiate the regenerative response affect the affinity, biosynthesis or degradation of the EGF receptor in hepatic cells using binding studies, specific immunoprecipitation and studies of receptor mRNA synthesis.